This might happen when visiting a site using HTTPS. Firefox does not have our certificate in its certificate “store”. (You can check the Firefox cert store in the Firefox advanced options). The easiest thing to do is close Firefox and then reset Replay Media Catcher's settings to the default (use the Reset button on the Settings page). RMC will restart, and when you start monitoring, the certificate will be added back into Firefox. You will be prompted that this has happened. You can then start Firefox and all should be good.